Welcome to the Forum!
I'll assume you're talking about the 7D rather than the d7.
I've seen posts on other forums where people had problems adjusting to the new AF system on the 7D, and it does take a little while to get used to it.
I have that combination and haven't notice it being soft. Here are a few shots I took at the zoo recently using that combination.
1/100s f/5.6 at 400.0mm iso400
1/125s f/5.6 at 310.0mm iso400
1/250s f/5.6 at 400.0mm iso400
Could you post a few pics with exif so we could possibly offer some help.